Traditional video imaging applications, including digital video, use colors that are defined using an additive display model that utilizes different relationships of red, green and blue lights for each pixel of a display, to represent different colors on the screen for each pixel. These red, green and blue lights may also be called “sub-pixels”. Typically the lights can range from a level of 0% (with no light emitted) to 100% (where the light is outputting as much light as possible). If all the red, green and blue lights are emitting 100% of their peak output for a particular pixel, then the pixel will appear to be “white”. If the red, green and blue lights for a particular pixel all emit 50% of their peak output, then the pixel will appear to be “gray”. If the red, green and blue lights for a particular pixel all emit 0% of their peak output, then the pixel will appear to be “black”. The range from “white” to “gray” to “black” is typically called the neutral or achromatic axis, since it is a range of lights that appear to lack any particular color and hue.
When the red, green and blue lights are all outputting 100% of their possible light output, the display will appear to be “white” at its maximum brightness. This “white” color is often called “display white”, and is the color of the “white” at maximum brightness. The color of the “display white” depends on the relationship between the efficiencies between the red, green and blue lights. For example, if the blue light is more efficient and can output more light than the red and green lights, then the “display white” at maximum brightness will look “bluish” and may often be described as “cool” white.
In reality, even neutral achromatic “white” has a small amount of color that may often be described as ranging from “warm” to “cool”. “Warm” neutrals are common when a scene is lit with tungsten-based lighting or candlelight, and have a slight yellow-orange color hue. “Cool” neutrals are common when a scene is lit with fluorescent lighting and have a slight bluish hue. The particular color cast or hue of the white in the scene is commonly referred to as the “creative white point”. It is common for most features and television to use a particular creative white point for the entirety of the work. Creative white points can often vary from title to title, but are usually in a range of whites defined by the names “D55” which is a warm white and “D65” which is a cool white. Creative white points typically are defined by the creative image content, and not the display or delivery system.
Home video distribution systems commonly use the Rec.709 video format. Video image data formatted for Rec.709 color space uses an “encoding white point” of D65, which means that when the encoded Rec.709 video signal contains equal amounts of red, green and blue video signal, a “D65” neutral color will be displayed to the viewer if the display system is functioning properly. If the video signal is at a 100% level in all red, green and blue channels, the color shown to the viewer will be D65 white. If the video signal is at 20% level in all red, green and blue channels, then the D65 gray color will be shown to the viewer. If other creative white points are used when the video content was mastered, then they can also be represented by the Rec.709 encoding, but the video signal values representing the red, green and blue channels will not have equal energy when non-D65 creative white points are used.
Color-difference transforms may be useful in imaging applications that support 4:2:2 or 4:2:0 subsampled image formats. Such transforms may allow images to be represented with lower uncompressed data bandwidths and also reduced compressed data rates. A color-difference transform is also useful in imaging applications as it acts as a de-correlation transform that removes redundancy that typically exists among the color components, thereby lowering the entropy of the transformed image data which can improve compression performance. Various color difference transforms are known in the art, but prior transforms may not provide optimal low-entropy output when the white point is other than a neutral or equal-energy value. More robust color transform techniques are therefore desirable.